1. Field of the Invention
This invention relates generally to systems for filling an implantable drug infusion pump. More particularly, the invention relates to a system for filling an implantable drug infusion pump whereby the pressure of the fluid delivered into the pump is controlled to avoid potentially damaging over-pressurization of the pump.
2. Description of the Related Art
The implantable drug infusion pump (IDIP) has provided physicians with a powerful tool for administering a wide variety of drugs and other agents, such as nerve growth factor, to very particularized sites within a patient's body, such as the intrathecal region of the spinal column. The IDIP has also freed some patients from the restrictions of typical intravenous drug infusion systems that typically include a wheeled cart that must be pulled around behind the patient.
An IDIP is ordinarily surgically implanted subcutaneously in the patient's abdomen. The IDIP has an internal reservoir for storing the drug or agent. After implantation, the drug or agent is delivered to a selected site in the patient's body via a catheter that is attached to the pump and tunneled subcutaneously to the selected site. Many medical applications calling for an IDIP require very minute dosages or drug or agent to be delivered to the selected site over a period of time. For example, dosages of 100 .mu.l over a span of twenty-four hours are not uncommon. IDIPs that are capable of delivering such minute quantities of drug or agent are sensitive to, and may be damaged by, over-pressurization of the reservoir.
Before the IDIP can be implanted in the patient's body, it must be filled with the applicable drug or agent. For some long-term applications, the IDIP may have to be refilled while the pump is still implanted within the patient's body. This is normally done by passing the drug or agent through a hypodermic needle that has been pierced through the patient's skin and coupled to the subcutaneously disposed IDIP. During these filling and refilling procedures, there is the risk that the pump's reservoir will become over-pressurized, possibly damaging the pump or affecting its performance.
Previous systems for filling or refilling an IDIP have utilized some type of pressure monitor to enable the technician to monitor the pressure of fluid delivered to the pump during filling. An example of such a prior art system is shown in FIG. 1. The prior art system 10 includes a filling syringe 12, a pressure monitor 14, a filling tube 16, and an IDIP 18. The inlet 20 of the pressure monitor 14 is coupled to the discharge outlet 22 of the filling syringe 12. The discharge orifice 24 of the pressure monitor is coupled to the inlet end 26 of the filling tube 16. As the plunger 28 of the syringe is depressed, drug flows from the syringe 12 through the pressure monitor 14 and the filling tube 16 and into the pump 18. The prior art system 10 has a notable disadvantage. Due to pressure loss in the system 10, the pressure monitor 14 will only indicate the pressure of the fluid in the pump 18 after the pump 18 has been filled and the flow of drug has stopped. Consequently, if the pump 18 has been accidentally over-filled, the pump may be over-pressurized and possibly damaged before the technician is made aware of the pressure level in the pump 18 by the pressure monitor 14. At that point, the pump 18 may be irreparably damaged.
The present invention is directed to overcoming the aforementioned disadvantage.